IMO a good focus tree could also increase play rate. I always want to play Japan more but the bad focus tree & boring naval mechanics turn me off.The German focus tree is solid, works and is probably the hardest to get right, because all major events require Germany as catalyst. I'd say they need a rework the least from a technical standpoint.
Japan, though? It really needs it, desperately, like yesterday.
That being said, from a standpoint of popularity the clear choice is Germany, the one that needs it the least. Funny how things sometimes work.
In terms of "how much better can you make the game with the same amount of work", Japan has to be the pick. Germany needs some polishing and lengthening of the focus tree, but it is still solid as you say.
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